1. Field of the Inventions
The present invention relates to the field of design of electronic circuits, and more specifically to a novel isolation cell used as an interface from a circuit portion operable in a power-down mode to a circuit portion in a power-up mode.
2. Related Art
There are often scenarios in which a portion of a circuit (“circuit portion”) is operated in a power-down mode (i.e., no supply of power), while some other circuit portions (modules) are operated in power-up mode. For example, at least to conserve powers, some circuit portions are operated in power-down mode when the circuit is expected not to be used for a short duration, while some other circuit portions continue to operate, for example, to determine when to bring back the circuit portions in power-down mode back to power-up mode or to maintain state information for continuity of operation.
Such a feature has been found to be particularly useful in CMOS sub-micron integrated circuit technology since CMOS transistors, even if not clocked, are known to leak current. As an illustration, in 130 nm process and above, power consumtion was often reduced by using a clock gating logic (which stops transitions on the clock signal), reduction of operating voltage, etc. In 90 nm process and below (i.e, as fabrication technology advances to compact more elements into smaller area), inherent leakage present with CMOS components is also a substantial component of the total power consumed, (in addition to the active power) and thus is generally not ignored. The leakage power becomes more significant due to ultra large scale integration of millions of transistors. Thus, by powering down unneeded circuit portions, leakage current may be avoided, thereby increasing the efficiency of power usage.
One recognized problem with concurrent operation of some circuit portions in power-up mode and other circuit portions in power-down mode is that a circuit portion in power-down mode may provide an input to the circuit portions in power-up mode on corresponding input terminals. In such a configuration, an input terminal of a power-up module can be in an undetermined state (floating) and may cause the circuit to malfunction, and thus undesirable as is well known in the relevant arts.
An approach to avoiding such undesirable states is to provide an isolation cell, which maintains a corresponding input terminal of a circuit portion operating in power-up mode (receiving signal from a circuit portion operating in power-down mode) at a desired logic level to avoid malfunctioning of the integrated circuit. The isolation cell is further generally designed to propagate the input signal to a corresponding module when/after the operation of the module transitions from the power-down mode to power-up mode.
Isolation cells need to be implemented with various requirements. One requirement is to provide flexibility to a designer to locate the isolation cell along with circuit portions which operate always in power-up mode or along with circuit portions which can operate in power-down mode as well. Locating an isolation cell along with a portion implies that both the isolation cell and the portion are co-powered from the same source such that both would operate together in either power-up mode or power-down mode. The absence of the flexibility to locate the isolation cell in either portion can lead to disadvantages such as constraints in routing, which in turn leads to larger area requirements and difficulty in matching timing requirement etc.
Another general requirement is to reduce power consumption by an isolation cell, particularly if the isolation cell is located with a circuit portion in power-up mode since the energy from the power source can be unnecessarily drained otherwise. At least in consumer electronics types of areas, avoiding power drainage is desirable. As a further illustration, the corresponding devices generally need to have power conservation modes (power management) during which the current drawn is extremely low of the order of tens of microamperes.
Hence what is needed is an isolation cell which meets one or more requirements such as the ones noted above.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.